This function will help you to add a custom thumbnail with get post meta in wordpress. See the code below.
<?php if( get_post_meta($post->ID, "picture", true)) : ?> <a title="Permanent Link to <?php the_title_attribute(); ?>" href="<?php the_permalink() ?>" > <img src="<?php echo get_post_meta($post->ID, "picture", true) ?>"/> </a> <?php endif;?>
Add the code in your template, where you would like to see the thumbnail. After inserting this code in your template, you need to add a custom field from your post. In the name filed of custom field give thumbnail and value should be the thumbnail image path. That’s it.